DE 10 2005 041 556 A1 discloses a method for determining an admission pressure prevailing between a master brake cylinder and an inlet valve of a wheel brake cylinder of a motor vehicle braking system. According to the described method, the admission pressure (generally the pressure at the outlet of the master brake cylinder) is determined taking into account the profile of a run-on voltage of an electric motor which is connected to a pump and which is actuated in a clocked fashion by means of a PWM (pulse width modulation) stage. For this purpose, a plurality of characteristic variables of the voltage profile are measured and are each used to determine an admission pressure value, wherein an evaluation of the quality and/or reliability of the measured characteristic variables, filtering and/or conditioning of the characteristic variables and/or of the admission pressure values determined therefrom in the event of deficient quality and/or reliability of the measured characteristic variables, and averaging of the admission pressure values determined from the various characteristic variables occurs, wherein only pressure values of a similar magnitude are taken into account, and averaging of the admission pressure values over time is carried out in order to damp fluctuations. For the later determination of the admission pressure during operation of the pump, a characteristic curve is stored in a data memory (for example read-only memory) of a motor vehicle braking system. The valves which are correspondingly used in the braking system for pressure control are then actuated taking into account the admission pressure calculated on the basis of the characteristic curve.
However, the implementation of the method described above is made more difficult by virtue of the fact that the characteristic curve is different for each individual pump and each individual braking system owing to fabrication tolerances. As the operating period of the pump increases, aging phenomena and wear phenomena, for example on the rotary bearing of the pump rotor, also occur and influence the operation characteristic of the brake system.
With respect to the determination of the admission pressure in a motor vehicle braking system it is therefore known to determine the rotational speed of the motor or of the pump from the generator voltage of an electric direct current motor which is connected to a hydraulic pump. The generator voltage can be provided, for example, by tapping the voltage at the motor terminals in the center of the inter-pulse period, wherein the voltage is sampled once per period with an A/D converter. It has become apparent that a motor rotational speed which is determined by means of a generator voltage which is obtained in this way is still too imprecise for determining the admission pressure in modern motor vehicle braking systems.
The object of the present invention is to make available a particularly precise method for estimating the set pressure without using a pressure sensor, wherein the method is advantageous, in particular, when the rotational speed of the pump motors used fluctuates individually or there is individual fabrication variation of the pump which leads to deviations of the delivery quantity.
This object is achieved according to the invention by means of the method as described herein.